1. Field of the Invention
This invention relates to an apparatus and method for atmospheric CVD (chemical vapor deposition) and, in particular, to and method for atmospheric CVD suitable for forming a thin film on an object e.g., a semiconductor wafer.
2. Description of the Related Art
FIG. 1 is a sectional view of the essential part of a conventional atmospheric CVD apparatus. The apparatus shown is equipped with a stage 3 for holding a heated semiconductor wafer 2 and a gas supply head 1 arranged over the stage 3. The semiconductor wafer 2 is heated by a resistance heater 4 incorporated in the stage 3. The gas supply head 1 is separated from the stage. 3 by a predetermined distance (e.g., 5 mm or less) and is equipped with a multitude of holes 7 and 8 through which a plurality of reaction gas components, such as SiH.sub.4 gas 5 and O.sub.2 gas 6 are respectively supplied. The SiH.sub.4 gas 5 and the O.sub.2 gas 6 are blown through these holes 7 and 8 to form a reaction product film 9 on the semiconductor wafer. Those portions of the SiH.sub.4 gas 5 and the O.sub.2 gas 6 which have not contributed to the formation of the reaction product film 9 as well as the reaction product gas generated through reaction in the gas phase by the reaction gases are discharged out of the atmospheric CVD apparatus by an exhaust flow 12 through an exhaust outlet 11 provided in an exhaust chamber 10.
With the above-described conventional atmospheric CVD apparatus, the semiconductor wafer 2 is arranged below the gas supply head 1, the reaction product film 9 being formed by supplying the SiH.sub.4 gas 5 and the O.sub.2 gas 6 blown out through the holes 7 and 8 to the surface of the semiconductor wafer 2 previously heated on the stage 3.
The growing speed of the reaction product film 9 in this type of atmospheric CVD apparatus is dependent on the concentration of the reaction gases supplied from the gas supply head 1 and the temperature of the semiconductor wafer 2, i.e., the temperature of the stage 3. Consequently, in order to ensure that the film thickness of the reaction product film 9 is uniform, the concentration of the reaction gases must be kept constant at all positions above the semiconductor wafer 2, and the temperature of the stage 3 must be controlled to ensure that it is kept at the level most suitable for the reaction.
With the above-described atmospheric CVD apparatus, setting the distance between the stage 3 and the gas supply head 1 at a value less than 5 mm, as shown in FIG. 1, will result in the SiH.sub.4 gas 5 and the O.sub.2 gas 6 impinging on the semiconductor wafer 2 before they are satisfactorily mixed together in the gas phase, and then being mixed together in a turbulent condition. As a result, the reaction gases supplied to the surface of the semiconductor wafer 2 is not mixed in an even concentration, so that a reaction product film 9 with a uniform film thickness cannot be obtained. In addition, the temperature of the reaction gases on the surface of the semiconductor wafer 2 is relatively low, so that no chemical reaction takes place on the surface of the semiconductor wafer 2, most of the gases undergoing reaction in the gas phase, which leads to the generation of foreign matter, i.e., unnecessary by-products.
When the distance between the stage 3 and the gas supply head 1 is set to a value as high as about 6 mm or more, as shown in FIG. 2, the reaction gas flow velocity must be greater than the velocity of the exhaust flow 12 (pumping speed) and the velocity of the gas flow generated by the temperature rise due to the heating of the stage 3. If the reaction gas flow velocity is low, the reaction gas components flow in the directions indicated by arrows 5 and 6, resulting in poor reaction efficiency on the semiconductor wafer 2. If, on the other hand, the reaction gas flow velocity is high enough for the reaction gas components, i.e., the SiH.sub.4 gas 5 and the O.sub.2 gas 6, to reach the semiconductor wafer 2 (as indicated by arrows 5' and 6' of FIG. 2), the concentration of the SiH.sub.4 gas 5 in those areas of the surface of the semiconductor wafer 2 which face the holes 7 becomes higher than in the other areas, and the concentration of the O.sub.2 gas in those areas of the surface of the semiconductor wafer 2 which face the holes 8 becomes higher than in the other areas, resulting in an uneven reaction gas concentration. Furthermore, since the reaction gas flow velocity is relatively high, the heat of the surface of the semiconductor wafer 2 is apt to be carried away by the reaction gases, so that the temperature of the surface of the semiconductor wafer 2 becomes unstable, a condition which precludes formation of a reaction product film 9 with a uniform film thickness.